Dear Editor, Jain et al.[1] described two intriguing cases with horizontal gaze palsy and progressive scoliosis. In the article, bilateral adduction on attempted lateral gaze was referred to by the term ‘synergistic convergence’. Bilateral deficit of abduction associated with simultaneous bilateral adduction on attempted lateral gaze has been previously referred to in different terms, including ‘synergistic convergence’ and ‘substituted convergence’.[2–4] Synergistic convergence, the term used when miosis is not present during convergence on attempted lateral gaze, has been related to peripheral causes, such as, congenital fibrosis of extraocular muscles, ocular misinnervation, or rarely, central causes like horizontal gaze palsy with progressive scoliosis, and brain stem dysplasia.[2] The term ‘substituted convergence’ has been used in earlier case reports for a convergence movement accompanied by miosis on attempted lateral gaze, in patients with pontine, cerebellar or posterior fossa lesions, and brain stem maldevelopments.[3] Presence of miosis during lateral gaze, in patients with simultaneous bilateral adduction on an attempted lateral gaze, is important for distinguishing the mechanism of this abnormal ocular movement. In the report of Jain NR et al.,[1] pupillary response to the lateral gaze in case 2 should have been pointed out, in order to understand the mechanism and to use the term ‘synergistic convergence’. The mechanism of synergistic convergence is controversial. Khan et al.[2] found normal convergence with appropriate miosis in their patient, whereas, miosis did not accompany convergence during attempted horizontal gaze; therefore, he pointed out discrete neuroanatomic localization of the three components of near triad (convergence, miosis, and accommodation). The significance of synergistic convergence was the rare glimpse of vergence dissociated from miosis, which suggested a more peripheral localization, selectively affecting vergence without pupillary involvement compared to substitution convergence. The mechanism of substituted convergence is also controversial. Leigh and Zee[4] offered a mechanism for this abnormal ocular movement in the setting of horizontal gaze, suggesting synchronized coexistance of the vergence and version system in conjugate eye movement. Lesioning of the version system can result in overactivity of the convergence system. The mechanism of convergence substitution in cases with horizontal gaze palsy with progressive scoliois (HGPPS) may be different from cases with acute pontine lesions, as presumably there is no acute disruption or injury to the underlying circuitry that is intact in convergence, but aberrant in conjugate horizontal gaze in HGPPS. The presence of miosis during lateral gaze suggests the use of the full vergence system in the absence of the version system to assist the limited gaze (adaptive phenomenon). Also, checking the amount of accommodation (myopic refractive error shifts) during substituted convergence movements should also have been determined, to confirm the use of convergence in these cases. In conclusion, the terms ‘synergistic’ or ‘substituted’ convergence should be used separately and further observations are needed to clarify the pathomechanism of those paradoxical eye movements in patients with HGPPS.